New York City wants to catch and store rainwater temporarily in new roof systems to stop heavy storms sending sewage spilling into city waterways.

The catchment systems would consist of "blue" roofs that have a series of drainage pools and "green" or grass- or ivy-covered roofs, under a plan unveiled by Mayor Michael Bloomberg.

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Bloomberg estimates the city could save $2.4 billion over 20 years if the state allows it to use this kind of green technology instead of relying on so-called grey infrastructure, such as storage tanks and tunnels.

"Our PlaNYC goal of making 90 percent of City waterways suitable for recreations requires us to do more, and that means reducing the combined sewer overflows that have plagued the City for decades," Bloomberg said in a statement.

During heavy storms, the city's 14 wastewater treatment plants turn into major polluters. That is because much of the city's water system was built 150 years ago when it was common practice to let rainwater drain into the sewage system.

To prevent treatment plants from flooding, bypasses kick in when there are major rainfalls, spewing sewage into harbors, canals and rivers.

New York City could capture an inch of rain in 10 percent of the older neighborhoods by using a variety of green methods, such as rain barrels and porous parking lots. Sidewalks could be planted with strips of greenery which also could absorb rainwater and release it slowly.